I'm looking for a leaf filter/separator or something else for the downpipe (also homemade, if that helps) that is quick and easy to clean/change and doesn't need to change/cut anything on the existing pipe or anything (access to the beginning of the pipe is hard to come by). Are there any other options?
Advice a good downpipe leaf/dirt filter
In front of the last hole you can reach, you can bend a hexagonal mesh called a rabbit screen.
Helps on my balcony against mice striving up and leaves striving down the drain.
I would consider replacing part of the downpipe with an appropriate dirt collector part. Perhaps hook up a rain barrel to it.
You should be able to find a proper fit under the mudguards or alternate leaf separators – but you can't install them without modifying the downpipe.